1. Field of the Invention
This invention is directed to a cathode paste containing a conductive polymer to replace carbon as the conductive material, and in particular, a cathode containing a cathodic material, a conductive polymer as the electroconductive material, a solid matrix forming monomer, a solvent and a viscosifier.
This invention is further directed to solid electrolytic cells (batteries) containing an anode, a solid electrolyte and a cathode containing a conductive polymer in place of carbon as the conductive material.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating a salt are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features. Notwithstanding their advantages, the manufacture of these solid batteries requires careful process controls to maximize the adherence of the various layers during formation of the electrolytic cells. Poorly adhered laminates can inhibit battery performance and can significantly reduce charge and discharge capacity.
Specifically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable salt, such as an inorganic ion salt, as a separate component. The inorganic matrix may be non-polymeric, e.g, .beta.-alumina, silver oxide, lithium iodide, and the like, or polymeric, e.g., inorganic (polyphosphazene) polymers, whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, ethylene oxide, propylene oxide, ethyleneimine, epichlorohydrin, ethylene succinate, and an acryloyl-derivatized alkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O--where R' is hydrogen or a lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte containing a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which is typically added to the matrix primarily to enhance the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
The solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of a salt, such as an inorganic ion salt, and the electrolyte solvent (typically a glyme compound and an organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate, e.g., the surface of a cathode, and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the salt and the solvent. This mixture is then placed on the surface of a suitable substrate, e.g., the surface of the cathode, and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, and the like) so as to form the solid, solvent-containing electrolyte.
Typically, cathodes for solid electrolytic cells are prepared by coating a mixture of cathodic material, an electroconductive material such as carbon, a solid matrix forming monomer, a solvent, and a viscosifyier on a current collector substrate followed by curing with e-beam or UV radiation. In many cases it has been found that such mixtures are very difficult to apply at room temperature as a smooth even coating.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery, i.e., an electrolytic cell.
Regardless of which of the above techniques is used in preparing the electrolytic cell, improvements are sought in the processability of the cathode paste. Improvements in the conductivity of the components of the cathode paste are also sought to improve the conductivity of the solid cathode. Improvements in the cathode paste components are further sought in the application of lightweight materials which can be cast into a smooth even coating or film.
In view of the above, the art is searching for methods to improve cathode manufacture, conductivity and coatability as well as to increase the adherence of the laminate layers of solid batteries employing such cathodes.